Magnetic oscillator

ABSTRACT

According to one embodiment, a magnetic oscillator includes a layered film and a pair of electrodes. The layered film includes a first ferromagnetic layer, an insulating layer stacked on the first ferromagnetic layer, and a second ferromagnetic layer stacked on the insulating layer. The pair of electrodes is configured to apply a current to the layered film in a direction perpendicular to a film surface of the layered film. Regions having different resistance area products are provided between the first ferromagnetic layer and the second ferromagnetic layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation Application of PCT Application No.PCT/JP2009/066970, filed Sep. 29, 2009, the entire contents of which areincorporated herein by reference.

FIELD

Embodiments described herein relate generally to a magnetic oscillator.

BACKGROUND

It is known that a microwave signal of a steady state, which responds toa direct current, can be generated by using spin transfer effect whichoccurs in a magnetic multilayer film of nanometer scale (for example,see S. I. Kiselev et al. “Microwave oscillations of a nanomagnet drivenby a spin-polarized current” Nature 425, 380 (2003)). The origin of themicrowave signal is magnetization oscillation of a magnetizationoscillation part in a magnetic multilayer film. In an experiment, in acurrent-perpendicular-to-plane (CPP) giant-magnetoresistive (GMR) effectfilm and a magnetic tunnel junction (MTJ) film, when the current densityexceeds the order of 10⁷A/cm², steady magnetization oscillation of highfrequency (GHz) is detected.

Microwave generators using spin transfer effect generated in a magneticmultilayer film are called spin transfer oscillators, magneticoscillators, and spin transfer oscillators. By a remarkably-advancedfine processing technology, it has become possible to process a CPP-GMRfilm and a magnetic tunnel junction film in a submicron size of about100 nm×100 nm. Magnetic oscillators are expected to be applied to minutemicrowave sources and resonators, and have been actively researched as aresearch of spintronics. The frequency of a microwave signal generatedfrom a magnetic oscillator depends on a current, and a magnetic fieldwhich acts on magnetization of a magnetization oscillation part in amagnetization multilayer film. In particular, by using its magneticfield dependence that the magnetization oscillation frequency changesaccording to the magnetic field, it has been proposed to apply magneticoscillators to magnetic sensors for an HDD which replace a GMR head anda TMR head (for example, see JP-A 2006-286855 (KOKAI)). When a magneticoscillator is used as a magnetic sensor for an HDD, the magnetic fieldof the HDD medium is sensed by detecting change in frequency caused bythe magnetic field.

Conventional magnetic oscillators have a structure in which a microwavesignal caused by oscillation of magnetization in a magnetoresistiveelement having a ferromagnetic multilayer film is taken out. Themagnetoresistive element has a three-layer structure including amagnetization free layer, a spacer layer, and a magnetization pinnedlayer, as a basic structure. When a direct current I flows through themagnetoresistive element by a power supply, the magnetization in themagnetization free layer is oscillated by a spin transfer effect betweenthe magnetization free layer and the magnetization pinned layer, and anangle θ between the magnetization of the magnetization free layer andthe magnetization of the magnetization pinned layer changes from momentto moment. With the change of the relative angle θ, the elementresistance changes from moment to moment mainly by magnetoresistiveeffect, and therefore an alternating-current component of the voltage isproduced. By extracting the alternating-current component of the voltageby a bias tee, a microwave signal is obtained.

A direct current I generated by a power source is not a desired value,but must be a current value which exceeds a threshold current value Icthat depends on the structure of the magnetoresistive element moduleincluding a ferromagnetic multilayer film and the surrounding magneticfield environment. Only when I>Ic is satisfied, magnetizationoscillation is induced in the magnetization free layer by the spintransfer effect. The value of the threshold current Ic is determined bya cross section of the magnetoresistive element and a threshold currentdensity value. It is known that the threshold current density value isabout 10⁷A/cm².

In the meantime, there is a quality (Q) factor as a quantity whichindicates a character of the oscillator. As an example of a Q-factor,there is mentioned an oscillating circuit which uses a crystaloscillator as a resonator. It is known that crystal oscillators have ahigh Q-factor of the order of 10⁶. An oscillating circuit which uses acrystal oscillator as a resonator achieves a Q-factor of the order of10³ to 10⁴, and obtains stable oscillation. The Q-factor is adimensionless quantity which is defined as follows, and a large Q-factormeans that oscillation is stable.

$Q = \frac{{energy}\mspace{14mu} {stored}\mspace{14mu} {in}\mspace{14mu} 1\mspace{14mu} {period}}{{power}\mspace{14mu} {loss}\mspace{14mu} {consumed}\mspace{14mu} {in}\mspace{14mu} 1\mspace{14mu} {period}\mspace{14mu} \left( {{dissipated}\mspace{14mu} {energy}} \right)}$

Oscillated state is often recognized by a frequency spectrum thereof,and in such a case the Q-factor is defined by Q f0/Δf. The symbol f0represents an oscillation frequency, and the symbol Δf represents a fullwidth at half maximum of an oscillation peak of the frequency spectrum.

A magnetic oscillator realized by a CPP-GMR film (hereinafter referredto as a “GMR oscillator”) is obtained when a spacer layer of themagnetoresistive element is formed of a non-magnetic metal layer such asCu. It has been known from experiment that oscillation of Q≈(10 GHz/1MHz) 10⁴ is obtained by a GMR oscillator (for example, see W. H. Rippardet al. “Current-driven microwave dynamics in magnetic point contacts asa function of applied field angle” Physical Review B 70, 100406 (R)(2004)). Specifically, GMR oscillators have performance which is greaterthan or equal to oscillating circuits which use a crystal oscillator asa resonator, with respect to the Q-factor. The reason why GMRoscillators can achieve a high Q-factor is that a large current can flowthrough GMR oscillators that are artificial metal lattices, all of whichare formed of metal material. It is known that a full width at halfmaximum Δf of the frequency spectrum is generally in inverse proportionto the square of current I (that is, Δf^(∝)1/I²). The value of Δfbecomes extremely small by flowing a large current, and thus a highQ-factor can be achieved. A high Q-factor is an advantage of GMRoscillators. GMR oscillators have, however, a disadvantage that a singleGMR oscillator outputs a weak electric power of the order of nanowatts(nW) at most, which is far from a practical electric power level ofmicrowatts (μW) and is not desirable for application. The reason why aGMR oscillator outputs a weak electric power of the order of nanowatt isthat GMR oscillators have a small magnetoresistive (MR) ratio of severalpercent at most. A structure of increasing an output power by arrangingGMR oscillators in an array has been proposed (for example, see S. Kakaet al. “Mutual phase-locking of microwave spin torque nano-oscillators”Nature 437, 389 (2005)). In the case of arranging GMR oscillators in anarray, however, it is necessary to arrange at least dozens of GMRoscillators in an array and synchronize all the oscillators with eachother, to increase the output power to a microwatt level. Therefore, itis difficult to manufacture the magnetic oscillator.

On the other hand, magnetic oscillators achieved by a magnetic tunneljunction film (hereinafter referred to as “TMR oscillators”) areobtained when a tunnel barrier is used as the spacer layer. In recentyears, high-quality magnetic tunnel junction films which have lowresistance and a high MR ratio have been developed, and expected to beapplied to spin injection magnetic random access memories (spin-RAM). Inparticular, it has been known by experiments that the MR ratio in a TMR(MgO-TMR) film which has a magnesium oxide (MgO) barrier is severalhundred percent or more. TMR oscillators can obtain large oscillationpower P since they have a high MR ratio. The oscillation power generatedby magnetic oscillators using an MgO-TMR film is actually coming nearpractical microwatt electric power level, and the maximum power levelwhich has been reported at present is 0.16 μW. It is impossible,however, to cause a large current to flow through magnetic oscillatorsusing a magnetic tunnel junction film such as an MgO-TMR film, unlikeGMR oscillators, due to the problem of insulation breakage by a tunnelbarrier, and thus it is difficult for the oscillators to realize a highQ-factor.

There are many cases where magnetization oscillation cannot be excitedin the first place in TMR oscillators. This is also due to insulationbreakdown of the tunnel barrier. This is because there are many caseswhere insulation breakdown is caused by a current which is smaller thanthe threshold current Ic, although magnetization oscillation is excitedin the free layer by the spin transfer effect only when I>Ic issatisfied as described above.

JP-A 2009-194070(KOKAI) discloses a complex magnetic oscillator which isobtained by magnetostatic-coupling an oscillation driving module formedof a GMR oscillator with an output module formed of a TMR oscillator,with good use of characters of GMR oscillators and TMR oscillators. Itis necessary, however, to manufacture the two oscillators very close toeach other, that is, 300 nm or less, to perform magnetostatic coupling,and thus the manufacturing process is difficult both in a planarstructure and a layered structure.

As described above, each of GMR oscillators and TMR oscillators havemerits and demerits. The merit of GMR oscillators is a high Q-factor,and the demerit thereof is small oscillation power. The merit of TMRoscillators is large oscillation power, that is, high output power, andthe demerit thereof is a low Q-factor.

Therefore, it is required for magnetic oscillators to have theadvantages of GMR oscillators and TMR oscillators, that is, a highQ-factor and high output power.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a magnetic oscillator according toembodiments.

FIG. 2 is a cross-sectional view illustrating a magnetic oscillatoraccording to a first embodiment.

FIG. 3 is a cross-sectional view illustrating a planar shape of themagnetic oscillator illustrated in FIG. 1.

FIG. 4 is a cross-sectional view illustrating a magnetic oscillatoraccording to a second embodiment.

FIG. 5 is a cross-sectional view illustrating a magnetic oscillatoraccording to a third embodiment.

FIG. 6 is a cross-sectional view illustrating a magnetic oscillatoraccording to Example 1.

FIG. 7 is a schematic diagram illustrating a power spectrum measurementsystem in the magnetic oscillator illustrated in FIG. 6.

FIG. 8 is a perspective view of a magnetic recording and reproducingapparatus according to the embodiments.

FIG. 9 is a perspective view of a magnetic head assembly illustrated inFIG. 8, as viewed from the magnetic disk side.

DETAILED DESCRIPTION

In general, according to one embodiment, a magnetic oscillator includesa layered film and a pair of electrodes. The layered film includes afirst ferromagnetic layer, an insulating layer stacked on the firstferromagnetic layer, and a second ferromagnetic layer stacked on theinsulating layer. The pair of electrodes is configured to apply acurrent to the layered film in a direction perpendicular to a filmsurface of the layered film. Regions having different resistance areaproducts are provided between the first ferromagnetic layer and thesecond ferromagnetic layer.

The embodiment provides a magnetic oscillator of high O-factor and highoutput power. The magnetic oscillator of the embodiment can be used fora microwave source, a resonator, and a magnetic sensor or the like.

Magnetic oscillators according to embodiments will be explainedhereinafter with reference to the accompanying drawings. In theembodiments, like reference numbers denote like elements, andduplication of explanation will be avoided. Each drawing is a schematicdiagram, and the illustrated shape, dimension and ratio include partsdifferent from those of the actual oscillator. When the oscillator isactually manufactured, they can be properly changed in consideration ofthe following explanation and publicly known art.

FIG. 1 illustrates a schematic structure of a magnetic oscillatoraccording to one embodiment. As illustrated in FIG. 1, the magneticoscillator is formed to have a layered structure obtained bysuccessively stacking a first ferromagnetic layer 1, an insulating layer(also referred to as tunnel barrier layer) 2, and a second ferromagneticlayer 3. Although, in FIG. 1, the first ferromagnetic layer 1 is amagnetization pinned layer whose magnetization is fixed and the secondferromagnetic layer 3 is a magnetization free layer whose magnetizationis not fixed, the structure is not limited to it. As one example, thefirst ferromagnetic layer 1 may be a magnetization free layer, and thesecond ferromagnetic layer 3 may be a magnetization pinned layer.Alternatively, both of the first and second ferromagnetic layers 1 and 3may be a magnetization free layer. In a magnetization free layer,direction of magnetization is changed in accordance with the externalmagnetic field. In addition, in the example illustrated in FIG. 1, themagnetization of the second ferromagnetic layer 3 is maintained by theexternal magnetic field in a direction which is antiparallel with thedirection of the magnetization of the first ferromagnetic layer 1, toeasily generate magnetization oscillation caused by the spin transfereffect. The following explanation mainly shows the example in which thefirst ferromagnetic layer 1 illustrated in FIG. 1 is a magnetizationpinned layer, and the second ferromagnetic layer 3 is a magnetizationfree layer.

The first and second ferromagnetic layers 1 and 3 are formed of Co, Ni,or Fe, or alloy which includes at least one of them. At both end partsof at least one of the first and second ferromagnetic layers 1 and 3, apair of bias magnetization films may be provided which apply a biasmagnetic field. One of the first and second ferromagnetic layers 1 and 3may be an exchange coupled film which is obtained by stacking aferromagnetic layer which has in-plane magnetic anisotropy and anantiferromagnetic layer. Alternatively, one of the first and secondferromagnetic layers 1 and 3 may be an exchange coupled film which isobtained by stacking a ferromagnetic layer which has in-plane magneticanisotropy, a nonmagnetic intermediate layer which controls themagnitude of the bias magnetic field, and an antiferromagnetic layer.Alternatively, one of the first and second ferromagnetic layers 1 and 3may be an exchange coupled film which is obtained by stacking anartificial ferrimagnetic film which has in-plane magnetic anisotropy andan antiferromagnetic layer.

The insulating layer 2 is formed of magnesium oxide (MgO) film, aluminumoxide (AlO) film, or the like. A magnetic oscillator using MgO film asthe insulating layer 2 has a large magnetoresistive (MR) ratio, and thuscan obtain high output power.

The magnetic oscillator illustrated in FIG. 1 includes a pair ofelectrodes, that is, a lower electrode 42 and an upper electrode 43,which apply a direct current I to a layered film 4 including the firstferromagnetic layer 1, the insulating layer 2, and the secondferromagnetic layer 3, in a direction perpendicular to a film surface ofthe layered film 4. The direction corresponds to a stacking direction inthe layers 1, 2, and 3 are stacked. The direct current I is suppliedfrom a power supply 6. The direct current I flows in a directionperpendicular to the film surface of the layered film 4, that is, fromthe second ferromagnetic layer 3 to the first ferromagnetic layer 1through the insulating layer 2. The intensity of a tunnel current whichflows through the insulating layer 2 depends on an angle between themagnetization of the first ferromagnetic layer 1 and the magnetizationof the second ferromagnetic layer 3. When the direct current I flowsthrough the layered film 4, the magnetization in the secondferromagnetic layer 3 is steadily oscillated by the spin transfer effectbetween the first and second ferromagnetic layers 1 and 3. Morespecifically, when the direct current I flows from the secondferromagnetic layer 3 toward the first ferromagnetic layer 1, electronsflow from the first ferromagnetic layer 1 toward the secondferromagnetic layer 3, and electrons which are spin-polarized by themagnetization of the first ferromagnetic layer 1 are injected into thesecond ferromagnetic layer 3. Then, the spin-polarized electrons act onthe magnetization of the second ferromagnetic layer 3, and precession ofthe magnetization of the second ferromagnetic layer 3 is induced.

In the case where both the first and second ferromagnetic layers 1 and 3are magnetization free layers, when the direct current I flows throughthe layered film 4, precession of the magnetizations of the first andsecond ferromagnetic layers 1 and 3 is induced with a fixed differencein phase between them.

The steady oscillation of the magnetization of the second ferromagneticlayer 3 becomes voltage oscillation by the magnetoresistive effect. Morespecifically, when the magnetization of the second ferromagnetic layer 3is oscillated by supply of the direct current I, the relative anglebetween the magnetization of the first ferromagnetic layer 1 and themagnetization of the second ferromagnetic layer 3 changes from moment tomoment. With change of the relative angle, the resistance of theoscillator changes from moment to moment mainly due to themagnetoresistive effect. Consequently, an alternating-current componentis produced in a voltage between the lower electrode 42 and the upperelectrode 43. The alternating-current component of the voltage isextracted by a bias tee 7 which is formed of a capacitor and aninductance, and a microwave signal (also referred to as a high-frequencyvoltage) P is obtained as output. The number of vibrations of thehigh-frequency voltage is equivalent to the number of vibrations of themagnetization oscillation, and depends on the size and thickness of themagnetization free layer, the direct current, and the magnitude of theexternal magnetic field. The thickness is defined in the stackingdirection. When the magnetic oscillator illustrated in FIG. 1 is usedfor a magnetic recording and reproducing apparatus explained later, themagnetic oscillator can detect continuous change of the number ofvibrations (that is, frequency) of the high-frequency voltage inaccordance with the magnetic field from the recording bits of therotated magnetic disk, and read out information of the recording bits.

To induce magnetization oscillation, it is necessary that the directcurrent I from the power supply 6 has a current value which exceeds athreshold current Ic (that is, I>Ic). The threshold current Ic dependson the structure of the layered film 4 and the surrounding magneticfield environment. The threshold current Ic is determined by thethreshold current density and a cross section of the layered film 4, ina plane which is perpendicular to the stacking direction. Therefore,generally, to oscillate the magnetization, it is a necessary conditionthat a current density J in the layered film 4 exceeds a thresholdcurrent density Jc.

The magnetic oscillator illustrated in FIG. 1 includes a region 51 whichhas a high resistance area product (RA) (hereinafter referred to as a“high RA region”), and a region 52 which has low resistance area product(hereinafter referred to as a “low RA region”), between the first andsecond ferromagnetic layers 1 and 3. The term “resistance area product”indicates a resistance per unit area (for example, 1 μm²) in a planeperpendicular to the direction of flow of the current. The resistancearea product of the high RA region 51 is denoted by RA1, and theresistance area product of the low RA region 52 is denoted by RA2. RA1is higher than RA2. When a constant current I is supplied to themagnetic oscillator which includes the high RA region 51 and the low RAregion 52, a voltage between the first and second ferromagnetic layers 1and 3 is fixed, and thus the ratio of the current density of the currentwhich flows through the low RA region 52 to the current density of thecurrent which flows through the high RA region 51 is determined by theratio of RA of the region 52 to RA of the region 51. Specifically, acurrent density J2 in the low RA region 52 is (RA1/RA2) times as largeas a current density J1 in the high RA region 51. In such a case, acurrent of high current density flows through the low RA region 52, andthe magnetization of the second ferromagnetic layer 3 is stronglyexcited. Since the second ferromagnetic layer 3 extends over the low RAregion 52 and the high RA region 51, the magnetization of the high RAregion 51 is excited by spin waves, and magnetization oscillation whichis larger than that caused by excitation of the magnetizationoscillation by spin torque in the current density J1 is excited in thehigh RA region 51. Specifically, in the second ferromagnetic layer 3,the magnetization of the low RA region 52 is strongly excited,oscillation of the magnetization of the low RA region 52 is transmittedto the magnetization of the high RA region 51 by exchange interaction,and the magnetization of the high RA region 51 is strongly excited. As aresult, large change in resistance is generated, and high output poweris obtained. In an extreme case, even when the current density does notexceed the threshold in the high RA region 51, the magnetic oscillatoraccording to the present embodiment can oscillate the magnetization ofthe second ferromagnetic layer 3 and obtain high output power, as longas a current density of the threshold value or more is achieved in thelow RA region 52.

A resistance R1 of the high RA region 51 is represented by theexpression R1=RA1/S1, by using an area S1 of the high RA region 51 andthe resistance area product RA1 of the high RA region 51. In the samemanner, a resistance R2 of the low RA region 52 is represented by theexpression R2=RA2/S2, by using an area S2 of the low RA region 52 andthe resistance area product RA2 of the low RA region 52. A resistance Rbetween the first and second ferromagnetic layers 1 and 3 is representedby the expression R=(R1×R2)/(R1+R2), and a voltage V is represented bythe expression V=1×(R1×R2)/(R1+R2). In addition, as described above, thecurrent density J2 in the low RA region 52 is (RA1/RA2) times as largeas the current density J1 in the high RA region 51. Therefore, as thedifference between the resistance area products RA1 and RA2 increase andthe area of the low RA region 52 decreases, the current density of thecurrent which flows through the low RA region 52 increases, and themagnetization of the second ferromagnetic layer 3 is oscillated morestrongly. As described above, the magnetic oscillator illustrated inFIG. 1 can realize high output power and a high Q-factor, by achieving apartly large current density in the layered film 4 having a high MRratio.

First Embodiment

A magnetic oscillator according to a first embodiment will be explainedhereinafter with reference to FIG. 2 and FIG. 3.

FIG. 2 illustrates a schematic structure of the magnetic oscillatoraccording to the first embodiment, and FIG. 3 illustrates a planar shapeof a layered film 4 illustrated in FIG. 2, as viewed from the stackingdirection. The magnetic oscillator illustrated in FIG. 2 includes alayered film 4 including a first ferromagnetic layer 1, a secondferromagnetic layer 3, and an insulating layer 2 that is interposedbetween the ferromagnetic layers 1 and 3, like the magnetic oscillatorillustrated in FIG. 1. The first and second ferromagnetic layers 1 and 3and the insulating layer 2 have the same planar shape, and form thelayered film 4. In the first embodiment, magnetization of the secondferromagnetic layer 3 is controlled by applying an external magneticfield H, such that the magnetization of the second ferromagnetic layer 3is antiparallel with a direction of magnetization of the firstferromagnetic layer 1. As illustrated in FIG. 3, a cross sectional shape(or a planar shape) of the layered film 4 in a plane perpendicular to astacking direction, is formed such that the cross sectional shape has atleast one edge having a large curvature or the cross sectional shape istapered as it goes toward a direction opposite to the direction of themagnetization of the first ferromagnetic layer 1. For example, the crosssectional shape of the layered film 4 is formed in a shape obtained byreplacing sides and vertices of an acute-angled isosceles triangle withcurves. When the external magnetic field H is applied such that themagnetization of the second ferromagnetic layer 3 is directed to theedge having a large curvature, the magnetization around the vertex ismore stabilized in a direction running along the edge than in adirection of the external magnetic field, and thus the magnetization iswarped by the effect of the shape. FIG. 3 schematically illustratesspatial distribution of the magnetization in the second ferromagneticlayer 3 by arrows.

When the magnetization of the first ferromagnetic layer 1 is fixed to beantiparallel with the magnetization of the second ferromagnetic layer 3,the resistance area product RA is reduced due to the MR effect, in theregion where the magnetization of the second ferromagnetic layer 3 islocally warped. When a direct current I flows through the layered film4, a low RA region 52 which has a low resistance area product has alocally high current density. When the magnetic oscillator ismanufactured with an insulating layer 2 having a very small thickness of1 nm or less and a large current is supplied to the magnetic oscillatorfor a long time with a voltage which does not exceed an insulationbreakdown voltage, a very small leak path 60 is formed in the low RAregion 52 by electromigration and soft breakdown. Although a resistancearea product RA in the leak path 60 is at least a digit smaller than theresistance area product in the insulating layer 2, the leak path 60 hasa very small area of several square nanometers and has high resistance.Therefore, most of the current flows through a high RA region 51 otherthan the leak path 60, and the MR ratio of the whole magnetic oscillatoris reduced. However, the MR ratio of the whole magnetic oscillator ismaintained at few score percent, which is a very high value incomparison with that of GMR elements. Since a current of a currentdensity which is at least a digit higher than that in other regionsflows through the leak path 60, the magnetization of the secondferromagnetic layer 3 is very strongly excited and oscillated around theleak path 60. Therefore, a high Q-factor is achieved in the magneticoscillator. In addition, the magnetic oscillator can obtain high outputpower, since the region other than the leak path 60 generates largeresistance change.

Second Embodiment

A magnetic oscillator according to a second embodiment will be explainedhereinafter with reference to FIG. 4.

FIG. 4 illustrates a schematic structure of the magnetic oscillatoraccording to the second embodiment. In the magnetic oscillatorillustrated in FIG. 4, an insulating layer 2 which is interposed betweenfirst and second ferromagnetic layers 1 and 3 has different in-planethicknesses. Since resistance (tunnel resistance) of an insulating layer4 changes according to the thickness of the insulating layer 2, themagnetic oscillator has regions which have different resistance areaproducts in a plane between the first and second ferromagnetic layers 1and 3, in the magnetic oscillator having the above structure. When acurrent of high current density flows through a low RA region 52, andmagnetization of the second ferromagnetic layer 3 in the low RA region52 is strongly oscillated. Oscillation of the magnetization in the lowRA region 52 is transmitted to a high RA region 51 by exchangeinteraction, magnetization of the high RA region 51 is also stronglyoscillated, and consequently large resistance change is generated.Therefore, the magnetic oscillator according to the second embodimentcan obtain a high Q-factor and high output power.

Third Embodiment

A magnetic oscillator according to a third embodiment will be explainedhereinafter with reference to FIG. 5.

FIG. 5 illustrates a schematic structure of the magnetic oscillatoraccording to the third embodiment. As illustrated in FIG. 5, themagnetic oscillator includes a metal path 70, which electricallyconnects first and second ferromagnetic layers 1 and 3, on a sidewall ofa layered film. Fine processing of the magnetic film is generallyperformed by ion milling. It is possible to adhere removed atoms againto the processed cross section, by changing processing conditions suchas an ion acceleration voltage and an ion incident angle. After theoscillator is isolated by ion milling under conditions in which atomsare not adhered, ion milling is performed from a specific direction withrespect to the oscillator under conditions in which atoms are adheredagain, and thereby a minute metal path 70 which electrically connectsthe first and second ferromagnetic layers 1 and 3 can be formed on partof the sidewall of the oscillator. Specifically, the first and secondferromagnetic layers 1 and 3 are connected to each other via aninsulating layer 2 which has high resistance area product and the metalpath 70 which has low resistance area product. Therefore, it is possibleto obtain a high Q-factor and high output power in the third embodiment,like the first and second embodiments.

Example 1

FIG. 6 is a cross-sectional view of a magnetic oscillator according toExample 1, which corresponds to the first embodiment. As illustrated inFIG. 6, the magnetic oscillator was obtained by forming layers on aglass substrate 41 by using a sputtering device, forming an upperelectrode 43 and a lower electrode 42 by photolithography and ionmilling, and processing a layered film 4 by electron-beam lithographyand ion milling.

The first ferromagnetic layer 1 was formed of an exchange bias filmwhich is obtained by stacking an antiferromagnetic layer 11 formed ofIrMn and an artificial ferri-structure in which an intermediate layer 13formed of Ru is interposed between a ferromagnetic layer 12 formed ofCoFe and a ferromagnetic layer 14 formed of CoFeB, and has fixedmagnetization. An insulating layer 2 was formed of MgO, and a secondferromagnetic layer 3 was formed of CoFeB. A lower electrode 42 wasformed of Ta/Cu/Ta, an upper electrode 43 was formed of Au/Cu, and aninsulator 44 was formed of SiO₂. When a current flows through themagnetic oscillator having the above structure, precession ofmagnetization of CoFeB being the second ferromagnetic layer 3 isinduced.

In the magnetic oscillator illustrated in FIG. 6, a layered film 4 wasprocessed to have a cross section shape which is tapered toward an edgeas illustrated in FIG. 3. An external magnetic field 500 Oe was appliedto the layered film 4 in a direction toward the tapered edge(magnetization of the first ferromagnetic layer was fixed to beantiparallel), a current was supplied to the layered film, and thereby aleak path as illustrated in FIG. 2 was formed in the insulating layer 2.By forming the leak path in the insulating film 2, the resistance of theoscillator was reduced, and the MR ratio thereof was reduced to 63%.

A lead of the upper electrode 43 and a lead of the lower electrode 42 ofthe magnetic oscillator were designed to serve as coplanar guide(waveguide) having a characteristic impedance of 50Ω.

FIG. 7 illustrates a measurement system for oscillation power spectrumin the magnetic oscillator. As illustrated in FIG. 7, in the measurementsystem, a bias tee 103 is connected to a waveguide 101, which transmitshigh-frequency oscillation from a magnetic oscillator 10, through ahigh-frequency probe 102, an input end of an amplifier 104 is connectedto an output end of the bias tee 103, and a spectrum analyzer 105 isconnected to an output end of the amplifier 104. In addition, adirect-current power supply 106 is connected to the bias tee 102.

Before explanation of a measurement result of oscillationcharacteristics of the magnetic oscillator in the above Example 1,oscillation characteristics of a TMR oscillator in prior art will beexplained hereinafter as Comparative Example 1. Although the TMRoscillator according to the Comparative Example 1 is manufactured withthe same structure as that of the layered film illustrated in FIG. 6,the oscillator is processed to have an ellipsoidal planar shape (with asize of about 60 nm×120 nm). The TMR oscillator has a resistance areaproduct RA of 12 Ωμm², and an MR ratio of 130%. In measurement ofoscillation characteristics of the TMR oscillator of Comparative Example1, an external magnetic field of 300 Oe was applied to the TMRoscillator in a direction which is inclined by 10° from a direction inwhich magnetization of the first ferromagnetic layer and magnetizationof the second ferromagnetic layer are antiparallel with each other, anddirect current was made flow from the second ferromagnetic layer to thefirst ferromagnetic layer. When the current was gradually increasedunder the above condition, spin torque increased together with increasein current, and thus the full width at half maximum of the oscillationpeak decreased. When a current having a current density of 4.0×10⁶A/cm²was supplied, however, the full width at half maximum was only reducedto about 300 MHz. On the other hand, when a current was made flowthrough the magnetic oscillator according to Example 1, the full widthat half maximum was 84 MHz with a current density of 1×10⁶A/cm², andoscillation peak having a full width at half maximum that does notexceed 100 MHz was recognized. The peak frequency f was about 3 GHz inboth of the oscillators. In the magnetic oscillator in which a leak pathis formed in the insulating layer 2 locally has high current density,and has small full width at half maximum Δf, although it has smallaverage current density. Therefore, the magnetic oscillator has a highQ-factor which is defined by Q=f0/Δf.

Example 2

In a magnetic oscillator according to Example 2, which corresponds tothe third embodiment, a TMR film was manufactured by the same process asthat explained in Example 1, and the film was processed such that theoscillator had an ellipsoidal cross section shape (or an ellipsoidalplanar shape) of about 110 nm×150 nm. The TMR film had an RA of 14 Ωμm²,and an MR ratio of 110%. Next, the TMR film was overetched by ionmilling, and thereby an oscillator including a metal path wasmanufactured by re-adhesion of metal on a sidewall. The manufacturedoscillator had a resistance area product RA of about 5.2 Ωμm², and an MRratio of 10%. When an external magnetic field of 330 Oe was applied tothe oscillator in a direction of an easy axis (which is almostantiparallel with the first ferromagnetic layer 1) of the secondferromagnetic layer 3 to supply a current having a current density of4.2×10⁶A/cm², an oscillation peak was recognized around a frequency of4.4 GHz, and an output power of 310 pW was obtained with a line width of24 MHz. As the magnetic oscillator according to Comparative Example 2, anon-shorted oval oscillator which has a size of about 60 nm×120 nm andmanufactured by using a TMR film having an RA of 12 Ωμm² and an MR ratioof 130%. When an external magnetic field of 300 Oe was applied to themagnetic oscillator of Comparative Example 2 in a direction that isinclined by 10° from the easy axis to supply a current having a currentdensity of 4.0×10⁶A/cm², the full width at half maximum in the frequencypeak was about 300 MHz, the output power was 120 pW. An oscillator whichincludes a short path (metal path) has smaller line width, that is, ahigher Q-factor, since it is strongly oscillated, although it has alower MR ratio. Therefore, the oscillator with a short path can obtainhigher output power.

Next, a magnetic recording and reproducing apparatus according to anembodiment will be explained hereinafter with reference to FIG. 8 andFIG. 9.

FIG. 8 illustrates a schematic structure of a magnetic recording andreproducing apparatus 150 according to the embodiment. As illustrated inFIG. 8, the magnetic recording and reproducing apparatus 150 comprises amagnetic disk (magnetic recording medium) 151. The magnetic disk 151 isrotated in a direction of arrow A by a spindle motor that is attached toa spindle 152. An actuator arm 154 is held by a pivot 153 which isprovided near the magnetic disk 151. A suspension 155 is attached to adistal end of the actuator arm 154. A head slider 156 is supported on alower surface of the suspension 155. The head slider 153 is providedwith a magnetic head which includes one of the magnetic oscillators thatare explained with reference to FIG. 1 to FIG. 5. A voice coil motor 157is formed in a proximal end part of the actuator arm 154.

When the magnetic disk 151 is rotated, the actuator arm 154 is rotatedby the voice coil motor 157 and the head slider 156 is loaded onto themagnetic disk 151, an air bearing surface (ABS) of the head slider 156provided with the magnetic head is held with a predetermined floatingquantity from the surface of the magnetic disk 151. In this state,information recorded on the magnetic disk 151 can be read out.

The head slider 156 may be of a contact motion type in which the slidercontacts the magnetic disk 151.

FIG. 9 is an enlarged perspective view of a magnetic head assembly whichincludes the actuator arm 154 and a distal end part, as viewed from themagnetic disk side. A magnetic head assembly 160 includes an actuatorarm 155, and the suspension 154 is connected to one end of the actuatorarm 155. A head slider 153 which has the magnetic head including one ofthe magnetic oscillators explained with reference to FIG. 1 to FIG. 5 isattached to a distal end of the suspension 154. A lead line for writingand reading signals is drawn in the suspension 154, and the lead line164 is electrically connected to each electrode of the magnetic headincorporated in the head slider 153. The lead line 164 is connected toan electrode pad 165 of the magnetic head assembly 160.

According to the magnetic reproducing apparatus of the embodiment, it ispossible to read information that is magnetically recorded on themagnetic disk 151 with a high recording density, by the magnetic headincluding one of the magnetic oscillators explained with reference toFIG. 1 to FIG. 5.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

1. A magnetic oscillator comprising: a layered film comprising a firstferromagnetic layer, an insulating layer stacked on the firstferromagnetic layer, and a second ferromagnetic layer stacked on theinsulating layer; and a pair of electrodes configured to apply a currentto the layered film in a direction perpendicular to a film surface ofthe layered film; wherein regions having different resistance areaproducts are provided between the first ferromagnetic layer and thesecond ferromagnetic layer.
 2. The oscillator according to claim 1,wherein each of the first ferromagnetic layer, the insulating layer, thesecond ferromagnetic layer has a cross sectional shape taken along aplane perpendicular to a stacking direction, the cross sectional shapebeing tapered toward an end part, and the insulating layer is providedwith a leak path in the end part, the leak path having a smallerresistance area product than a resistance area product of the insulatinglayer.
 3. The oscillator according to claim 1, wherein the insulatinglayer includes the regions having different thicknesses.
 4. Theoscillator according to claim 1, further comprising a metal path whichelectrically connects the first ferromagnetic layer and the secondferromagnetic layer.
 5. A magnetic recording and reproducing apparatuscomprising: a magnetic oscillator comprising a layered film and a pairof electrodes, the layered film comprising a first ferromagnetic layer,an insulating layer stacked on the first ferromagnetic layer, and asecond ferromagnetic layer stacked on the insulating layer, the pair ofelectrodes configured to apply a current to the layered film in adirection perpendicular to a film surface of the layered film, whereinregions having different resistance area products are provided betweenthe first ferromagnetic layer and the second ferromagnetic layer.
 6. Theapparatus according to claim 5, wherein each of the first ferromagneticlayer, the insulating layer, the second ferromagnetic layer has a crosssectional shape taken along a plane perpendicular to a stackingdirection, the cross sectional shape being tapered toward an end part,and the insulating layer is provided with a leak path in the end part,the leak path having a smaller resistance area product than a resistancearea product of the insulating layer.
 7. The apparatus according toclaim 5, wherein the insulating layer includes the regions havingdifferent thicknesses.
 8. The apparatus according to claim 5, furthercomprising a metal path which electrically connects the firstferromagnetic layer and the second ferromagnetic layer.